Formation Of Domain Structure Research Articles

Shape-induced phenomena in finite-size antiferromagnets

It is of common knowledge that the direction of easy axis in the finite-size ferromagnetic sample is controlled by its shape. In the present paper we show that a similar phenomenon should be observed in the compensated antiferromagnets with strong magnetoelastic coupling. Destressing energy which originates from the long-range magnetoelastic forces is analogous to demagnetization energy in ferromagnetic materials and is responsible for the formation of equilibrium domain structure and anisotropy of macroscopic magnetic properties. In particular, crystal shape may be a source of additional uniaxial magnetic anisotropy which removes degeneracy of antiferromagnetic vector or artificial 4th order anisotropy in the case of a square cross-section sample. In a special case of antiferromagnetic nanopillars shape-induced anisotropy can be substantially enhanced due to lattice mismatch with the substrate. These effects can be detected by the magnetic rotational torque and antiferromagnetic resonance measurements.

BiFeO 3 thin films prepared by MOCVD

BiFeO 3 thin films were grown by metal organic chemical vapour deposition (MOCVD) at the temperature T = 700 °C using Fe(thd) 3, Bi(C 6H 5) 3 as volatile precursors. High thermal stability of Bi(C 6H 5) 3 makes the film stoichiometry very sensible to the deposition conditions, in particular to the precursor residence time in the reactor. We tested novel precursors Bi(thd) 3 and Bi(CH 3COO) 3 possessing lower thermal stability. They drive the process into the diffusion control regime when cation stoichiometry in the layer is easier to control. The possibility to use them for epitaxial growth of BiFeO 3 thin films was demonstrated. BiFeO 3 film forms the self-organized nanodomain variant structure with (110) out-of-plane orientation on (001) ZrO 2(Y 2O 3) substrate. The high resolution TEM images confirm variant domain structure formation. The ferroelectric nature of BiFeO 3 films was assessed at the room temperature by piezoelectric force microscopy (PFM).

Crystal structure of CuIn3Se5 and CuIn5Se8 ternary compounds

The structures of single crystals of CuIn3Se5 and CuIn5Se8 ternary compounds have been studied by X-ray diffraction for the first time. The unit cell parameters of CuIn3Se5 and CuIn5Se8 crystals are determined. It is established that CuIn3Se5 has a thiogallate lattice with c = 2a, which leads to the domain structure formation. CuIn5Se8 has a hexagonal lattice and exhibits cleavage along the (001) plane. Relationships between the atomic composition and crystal structure of these new ternary semiconductor compounds are discussed.

Formation of Self-Similar Surface Nano-Domain Structures in Lithium Niobate Under Highly Nonequilibrium Conditions

Formation of the nanoscale domain structure in the surface layer as a result of pulsed UV laser irradiation has been studied experimentally in congruent single crystalline lithium niobate LiNbO3. The self-assembled structures have been classified and statistically analyzed. The structure is formed by growth, turning, and branching of domain rays strictly oriented along three Y+ directions. High-resolution microscopy shows that the rays represent the chains of individual needle-like nano-domains. The results of computer simulation of the domain structure formation using experimentally revealed rules are in good agreement with the experimental ones.

Giant parametric amplification of the nonlinear response in a single crystal of beryllium in a quantizing magnetic field

The nonlinear response to an ac magnetic field with and without parametric pumping was studied experimentally on a rod-like beryllium single crystal at low temperatures in a quantizing magnetic field applied parallel to the hexagonal axis. At low temperatures (⩽3K) giant parametric amplification of the nonlinear response is observed in narrow windows near de Haas-van Alphen (dHvA) beating antinodes, where the amplitude of the magnetic oscillations is maximal. This effect is of threshold character with respect to the amplitude of the dHvA oscillations and occurs as a result of a diamagnetic phase transition of a strongly correlated electron gas into nonuniform state, with the formation of Condon domain structure. It is shown that the nonuniform phase appears periodically in a magnetic field. The components of the rectified signal field reproduce the envelope of H-T critical curves and can be used for reconstruction of diamagnetic phase diagrams.

Diamagnetic phase transition and phase diagrams in beryllium

The model of diamagnetic phase transition in beryllium which takes into account the quasi 2-dimensional shape of the Fermi surface of beryllium is proposed. It explains correctly the recent experimental data on observation of non-homogeneous phase in beryllium at the conditions of strong dHvA effect when the strong correlation of electron gas results in instability of homogeneous phase and formation of Condon domain structure.

The importance of arginine mutation for the evolutionary structure and function of phenylalanine hydroxylase gene

Phenylalanine hydroxylase ( PAH) gene mutations were investigated in 23 (46 alleles) unrelated phenylketonuria (PKU) patients in Cukurova region. First, all exons of PAH gene were screened by denaturing high performance liquid chromatography (DHPLC), and then, the suspicious samples were analyzed by direct sequencing technique. Consequently, the following results were obtained: IVS10-11g → a splicing mutation in 27/46 (58.7%), R261Q mutation in 7/46 (15.2%) and E178G, R243X, R243Q, P281L, Y386C, R408W mutations, each found in the frequency of 2/46 (4.3%). In many countries, Arginine mutations have the highest frequency among PAH gene mutations in PKU patients. Although, CpG dinucleotids are effective in mutations resulting in arginine changes, this finding originated from the studies on the causes of mutations rather than the studies on the importance of arginine amino acid. In our analyses, we have detected that a majority of mutations causing a change in arginine and other amino acids concentrated in exon 7 comprising the catalytic domain (residues 143–410) of PAH gene. Several studies has emphasized the role of arginine amino acid; with the following outcomes; arginine repetition is significant for RNA binding proteins, and for histon proteins in eukaryotic gene expression, and also arginine repetition occurring in the structure of signal recognition particle's (SRPs) as a consequence of post-translational processes is very important in terms of gene expression. Therefore, the role of arginine amino acid in PAH gene is rather remarkable in that it shows the role of amino acids in the protein/RNA interaction that has started in the evolutionary process and is still preserved and maintained in the motif formation of active domain structure due to its strong binding properties. Thus, such properties imply that both arginine amino acid and exon 7 is of great significance with regards to the structure and function of the PheOH enzyme.

On the theory of domain structure in ferromagnetic phase of diluted magnetic semiconductors

We present a comprehensive analysis of domain structure formation in ferromagnetic phase of diluted magnetic semiconductors (DMS) of p-type. Our analysis is carried out on the base of effective magnetic free energy of DMS calculated by us earlier [Yu.G. Semenov, V.A. Stephanovich, Phys. Rev. B 67 (2003) 195203]. This free energy, substituting DMS (a disordered magnet) by effective ordered substance, permits to apply the standard phenomenological approach to domain structure calculation. Using coupled system of Maxwell equations with those obtained by minimization of above free energy functional, we show the existence of critical ratio ν cr of concentration of charge carriers and magnetic ions such that sample critical thickness L cr (such that at L < L cr a sample is monodomain) diverges as ν → ν cr . At ν > ν cr the sample is monodomain. This feature makes DMS different from conventional ordered magnets as it gives a possibility to control the sample critical thickness and emerging domain structure period by variation of ν. As concentration of magnetic impurities grows, ν cr → ∞ restoring conventional behavior of ordered magnets. Above facts have been revealed by examination of the temperature of transition to inhomogeneous magnetic state (stripe domain structure) in a slab of finite thickness L of p-type DMS. Our theory can be easily generalized for arbitrary temperature and DMS shape.

Micromagnetic simulation of spin transfer torque switching by nanosecond current pulses

Magnetization reversal by spin transfer torque due to current flowing in magnetic tunnel junction nanopillar with 200nm×125nm elliptical cross section is studied by micromagnetic simulation assuming the Slonczewski model for spin transfer torque. The magnetization dynamics during the switching is found to be highly inhomogeneous with formation of complex domain structure and spin wave generation even for artificially increased exchange length. The Oersted field influence on onset of precession leading to switching is described in terms of spin wave generation. The switching time as a function of current is found to have nonmonotonic behavior, which is not completely understood but is found to be related to the presence of Oersted field.

Stress-induced domain structure formation in nanodimensional barium strontium titanate films

Thin heteroepitaxial films of BaxSr1−x TiO3 (BST) with x = 0.80 possessing high structural perfection have been grown by RF magnetron sputtering on MgO(001) substrates. The microstructure and crystallographic characteristics of the BST films of various thicknesses have been studied using electron microscopy and X-ray diffraction. It is established that there is a threshold thickness at which the BST crystal lattice parameter in the direction perpendicular to the substrate exhibits a sharp change.

The magnetic domain structure of DyFe 11Ti single crystals

The magnetic domain structure (DS) on (1 1 0)-, (0 0 1)- and arbitrary-oriented planes of DyFe 11Ti single crystals is investigated in the temperature region from room temperature down to 4.2 K. The role of magneto-elastic interaction and deviation from stoichiometric ratio 1:12 in DS formation and spin-reorientation transitions is discussed.

Microstructural study of epitaxial Zn1−xMgxO composition spreads

We have fabricated Zn1−xMgxO epitaxial thin-film composition spreads by layer-by-layer pulsed laser deposition (PLD) method where the composition across the chip is linearly varied from ZnO to MgO. In this paper we discuss the cross-sectional transmission electron microscopy study of the combinatorial spreads. The full orientation relationships between substrate and the ZnO and MgO phases were established. Formation of twin-related domain structure was found for the [111]-oriented MgO. Formation of the [100]-oriented cubic MgO in [0001]-oriented ZnO was observed in the mixed region in the middle of the spread. This relationship can potentially be used to grow [100]-oriented cubic structures epitaxially on c-axis-oriented hexagonal substrates. For the extended solid solution of ZnO, a high density of defects was found. Analysis of the defects established that they are translational domains formed by three-dimensional island growth of PLD and specific to the crystallography of ZnO. The ZnO phase appears without precipitation of the MgO phase, which supports the idea of extended substitutional solid solution.

Switching Processes and Formation of the Stable Artificial Domain Structure in Ferroelectric LaBGeO5

The processes of polarization switching in LaBGeO 5 (LBGO) crystals are studied in details for the triangle and pulse electric field in the wide temperature range from room temperature (RT) to Tc = 530°C. It is revealed that the temperature dependence of a coercive field (f = 0.1 Hz) has an exponential form and this field reaches a value 150 kV/cm at the RT. The temperature evolution of the switching time and the form of the switching current are obtained in the mode of the successive solitary pulses and in the steady conditions. The features of “non-classical” behavior are noted (the dependence of switching parameters on the prehistory of samples, frequency dependence of the switching time). It is shown that the stable artificial domain structure at RT with the periodicity of 200 μm can be obtained by applying about 300–350 V/mm near 300°C. The stable domain structure was contrasted by means of SHG microscope probing.

Modeling of twinning in epitaxial (001)-oriented La0.67Sr0.33MnO3 thin films

Twinning, i.e., the formation of structural domains, in La1−xSrxMnO3 thin films, which grow coherently on (001)-oriented cubic substrates, is explained as a result of shear strain relaxation within the distorted rhombohedral La1−xSrxMnO3 crystal lattice. A one-dimensional periodic model structure of twins is proposed and the nonuniform elastic strains within twinned La1−xSrxMnO3 films are calculated by applying the coherency-defect technique. The strain field depends on the ratio of the domain width and film thickness and exhibits maxima at the triple junctions of the domain boundaries and the film/substrate interface. The equilibrium domain width is derived as a function of the film thickness by minimizing the total energy of the system including contributions from elastic strain and domain-wall energy. From comparison of the theoretical results with recent electron microscope observations of twins the domain-wall energy can be predicted. From the observed average domain width in La0.67Sr0.33MnO3 films, the domain-wall energy of 1.35mJ∕m2 is suggested. The possibility of a preferred nucleation of the ferromagnetic phase at triple junctions due to spatial strain variations is discussed.

Nonuniform and sequential magnetization reversal via domain structure formation for multilayered system with grain size induced enhanced exchange bias

We report on the magnetization reversal in series of exchange-biased multilayers NiFe(10.0 nm)/[ Ir20Mn80(6.0 nm)/Co80Fe20(3.0 nm)] N studied by specular reflection and off-specular scattering of polarized neutrons. All specimens are sputtered and post-annealed at 530 K (i.e. above the IrMn Neel temperature of 520 K) in Ar atmosphere before cooling to room temperature in the presence of a field of 130 Oe which induces the unidirectional anisotropy. We find HEB is dependent upon the number of bilayers N as it gradually increases from 0.33 kOe for N=1 to a considerably higher value of upto ≈0.9 kOe for N=10. X-ray specular and diffuse scattering data reveal no significant variation of the lateral correlation length and only a weak dependence of the vertical rms interface roughness on N. Atomic and magnetic force microscopy, however, show a strong reduction of the grain size accompanied by distinct changes of the ferromagnetic domain structure. The enhancement of the exchange bias effect is presumably related to the shrinking of the related domain size in the antiferromagnet due to the structural evolution in the multilayers. Polarized neutron reflectometry (PNR) measurements are done at different applied fields sweeping both branches of the hysteresis loop. The spin-flip (SF) cross section of both the N=10 and 3 samples show diffusely scattered intensity appears gradually as the field approaches HEB and is most intense where the net magnetization vanishes. The disappearance of diffuse scattering in saturation indicates that the off-specular intensity is related to the reversal process. The reversal proceeds sequentially starting with the bottom (top) CoFe layer for decreasing (increasing) field and is related to the evolution of the grain size along the stack. The reversal of each CoFe layer is for both field branches due to domain wall motion. Thus as a main result, we observe a sequential and symmetric magnetization reversal in exchange-biased multilayers. The concomitant in-plane magnetization fluctuations revealed by off-specular spin-flip scattering indicate a more complex reversal mechanism than hitherto considered. Moreover, although the grain size decreases from N=3 to 10 by a factor of about four the reversal mechanism remains similar.

Strain-driven domain structure control and ferroelectric properties of BaTiO 3 thin films

We have studied the domain structure formation in annealed BaTiO 3 films grown on SrTiO 3. We show that it is possible to utilize the 2.28% lattice mismatch strain at the SrTiO 3/BaTiO 3 interface to obtain uniaxially c-axis-oriented BaTiO 3 films with a thickness of up to about 200 nm. This thickness is much larger than the typical critical thickness for BaTiO 3 films on SrTiO 3 and such films therefore can potentially be useful for optical waveguide applications. We also looked at the polarization properties of the uniaxial BaTiO 3 films. Our measurements and analysis indicate that the heavily distorted interface region, with a thickness of approximately 2 nm is non-ferroelectric.

Mechanism of Formation of the Equilibrium Domain Structure in Crystals Undergoing Thermoelastic Phase Transitions

The mechanism of formation of the equilibrium domain structure during a thermoelastic phase transition is proposed. This mechanism is related to long-range elastic strain fields created by “elastic charges” at the free crystal surface. It is assumed that, during a phase transition, there appears not only the nonzero primary (antiferromagnetic, martensitic) order parameter in the crystal but also an internal (quasi-plastic) stress rigidly related to the order parameter. The orientation of this stress with respect to the crystallographic axes can be changed by external fields. Elastic charges arise due to those components of the internal stress tensor whose flux across the crystal surface is nonzero. The nonlocal destressing energy is found. It is shown that, for a certain shape of a sample, an inhomogeneous distribution of the primary order parameter (a domain structure) is energetically more favorable. The characteristic field at which a sample becomes a single domain is shown to be dependent on the shape of the crystal.

On the theory of the formation of equilibrium domain structure in antiferromagnets

A model is proposed which describes the formation and reversible rearrangement of the equilibrium domain structure in bulk antiferromagnets with a rather strong magnetoelastic coupling. The model is based on the assumed existence of a microscopic ordering of a tensor nature—the microstress tensor that arises due to magnetoelastic coupling during the formation of the magnetic moment. It is necessary to take such a parameter into account for adequate description not only of the macroscopic internal stresses and the spontaneous strains corresponding to them but also of the microstructure of the crystal (e.g., the domain structure). The microstresses arising locally in each unit cell are equivalent, from a formal standpoint, to elastic dipoles, and they create long-range fields whose contribution to the free energy of the crystal is analogous to that of the magnetostatic energy in ferromagnets and favors a decrease in the macroscopic strain of the sample through the formation of an equilibrium domain structure. The corresponding contribution is given the name “destressing energy” by the authors. It is shown that taking this energy into account in antiferromagnetic crystals allows one not only to explain the cause of the formation of the domain structure but also to trace its dependence on the shape of the crystal and the external fields.

Reversible and irreversible changes of surface morphology by order–disorder transition in CuAu alloy

The change of symmetry from the disordered fcc structure to tetragonal or orthorhombic structure is accompanied in CuAu alloy by anisotropy of lattice parameters and also by local generation of c-variants of structural antiphase domains. Macroscopic results of these processes can be observed as a dynamic change of the surface morphology. Some surface changes are reversible, on the other hand the internal stresses connected with the order–disorder transformation are also responsible for irreversible surface deformation effects. The domain structure formation can be influenced by external load and a shape memory effect can be observed at special conditions in CuAu. A combination of in-situ microscopic video cinematography and post-mortem 3-D atomic force microscopy (AFM) has been used for the surface study. The AFM images have enabled a detailed analysis of the surface morphology and the cinematography has given an in-situ information dealing with conditions and kinetics of observed surface changes. Measurements on CuAu single- and poly-crystalline samples have been made for a wide variety of experimental conditions (heating/cooling rates, external load, thermal history of the sample).

The dynamics of the sandwich domain structure in 3%Si–Fe sheets

Two types of the domain walls motion equations have been derived in which a step-like movement character and polarization of the domain walls parallel to the magnetic sheet plane and forming a sandwich domain structure were take into account. The deduced equations permit to calculate the domain walls coordinates x ̄ (τ) in this structure and the damping coefficient β( x ̄ ) or the demagnetizing field H d ( x ̄ ) in each domain wall plane during one cycle of the dynamic magnetization reversal accompanied by the Matteucci emf appearance. The steps shape of the moving domain walls is non-essential for these equations. With the help of the derived equations it has been discovered that the interaction of the 3%Si–Fe two sheets by means of the transverse magnetic stray fields leads to formation of the sandwich domain structure in each sheet during dynamic (200 Hz) magnetization reversal. The appearance, the movement direction, the velocity and annihilation of the magnetization reversal fronts in each sheet are determined by local demagnetizing fields and by the magnetic fields of eddy currents. The first front appears in each sheet at the beginning of each semiperiod of the magnetization reversal close to the contact surface of sheets. In Δτ≈0.8 ms on an opposite surface of each sheet the second magnetization reversal front appears and it moves toward the first one. Before the collision of the fronts close to the sheet middle the first front changes the movement direction and both fronts move to the contact surface of the sheets. The simultaneous one way movement of the fronts is accompanied by the decrease of their mutual distance.